[Fozzie_DeBear]

Originally Posted by photon

How does the chlorine get out of the water? If the concentration of chlorine right before it goes into the system is 0.5 parts per million, how does that concentration go down as it goes through the pipes?

Good question, wiki has some answers

Quality standards in the United States require specific amounts of disinfectant (example, residual chlorine content) in the water after it leaves the WTP (Water Treatment Plant), at the end of the treatment process to reduce the risk of re-contamination while the water is in the distribution system.

Disinfection is normally the last step in purifying drinking water. Water is disinfected to kill any pathogens which pass through the filters. Possible pathogens include viruses, bacteria, including Escherichia coli, Campylobacter and Shigella, and protozoans, including G. lamblia and other Cryptosporidia. In most developed countries, public water supplies are required to maintain a residual disinfecting agent throughout the distribution system, in which water may remain for days before reaching the consumer. Following the introduction of any chemical disinfecting agent, the water is usually held in temporary storage - often called a contact tank or clear well to allow the disinfecting action to complete. is the sanitising of sterilisation of water. This is done by adding gaseous dissloved chlorine in the water. Chlorine at a concentration of 1 or 2 ppm destroys bacteria and some viruses. Sufficient chlorine is added to the water (with careful monitoring) to ensure that the concentration stays slightly above 1ppm until the water reaches the end user.